1. Field of the Invention
The invention relates to electrically operated friction devices and more particularly to electromagnetic clutches.
2. Description of the Prior Art
Electromagnetic clutches as used heretofore have a toothed driving clutch member and a toothed driven clutch member which are moved together upon energization of an electromagnet for engagement of their teeth. A spring disengages the teeth when the electromagnet is deenergized. However, residual magnetism in the driving and driven clutch members tends to impede the separation of the clutch members and a heavy spring is required for initial breakway from the metal to metal contact of the teeth. The same heavy spring is used to further move the clutch members a sufficient distance to separate the teeth and permit relative rotation of the clutch members. When it is desired to engage the clutches, the electromagnet is energized. The magnetic force thus produced must not only overcome the losses associated with maintaining the field across an air gap but also the heavy spring force during the entire movement of the clutch members to engage the teeth. This requires a larger electromagnet in order to compensate for the increase in electromagnetic force required to engage the clutch members.
In U.S. Pat. No. 4,010,832, a two stage spring force was devised in order to reduce the electromagnetic force required to engage the clutch members. In this device, a washer spring was formed with a stiff finger with a relatively small offset for initially separating the clutch members. In addition, the spring was also formed with a light finger with a relatively large offset for further separating the clutch members until the teeth are completely disengaged. Thus, the magnetic force required to engage the clutch members was reduced.
This device has been found to have significant drawbacks. The tooling required to form the spring with its two different offsets and fingers was very expensive. In addition, it has been found difficult to fabricate the spring offsets within the production tolerances required.